Aphid, Alternaria and Honeybees in Flowering Mustard
In Barpeta, a 0.6 ha mustard crop (var. M-27) is at 50% flowering. Scouting on 20 January shows 8–10 mustard aphids per terminal twig on 30% of plants, 3–4 ladybird beetles per plant and several mummified aphids. Leaves also show about 5% area covered with brown concentric lesions typical of Alternaria blight. The farmer has imidacloprid 17.8 SL, malathion 50 EC, mancozeb 75 WP and neem oil. Two honeybee colonies are kept at the field edge for the nearby litchi orchard, and the forecaster predicts dry weather with 12–14°C nights for the next week. The farmer wants a single tank-mix spray to 'finish both problems' before the weekend and asks whether it will be safe and effective.
1. Decide whether an insecticide spray for aphids is justified now. Show threshold reasoning and pollinator considerations.
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No insecticide spray is justified yet. The usual action threshold for mustard aphid is around 20–25 aphids per twig, or roughly 50% of plants infested. The field has 8–10 aphids per twig on only 30% of plants, with 3–4 ladybirds per plant and mummified aphids indicating active parasitism. The population is being held below the threshold by natural enemies.
Flowering is also the most pollinator-sensitive stage. Imidacloprid is systemic and highly toxic to bees, and malathion is a broad-spectrum insecticide that would kill ladybirds and parasitoids as well as aphids. Spraying now would risk pollination of both mustard and the nearby litchi, and would disrupt the biological control that is already working. The correct action is to re-scout every 3–4 days. If the count rises above about 25 aphids per twig and predator numbers fall, use a selective aphicide at dusk after bee foraging, never as a prophylactic tank-mix.
2. Design a management plan for the Alternaria blight that fits the flowering stage, and evaluate the farmer's proposed tank-mix.
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At 5% leaf area affected, a protectant fungicide is reasonable, but it should be sprayed as a targeted disease measure, not mixed with an unnecessary insecticide. Mancozeb 75 WP at about 2.0–2.5 g per litre of water, applied in the evening with good coverage of lower leaves, is appropriate; a second spray can follow after 10–12 days if weather remains favourable and symptoms progress. Use 400–600 litres of spray solution per hectare depending on canopy. Observe the label pre-harvest interval.
The proposed four-way tank-mix is poor practice. It combines a bee-toxic systemic insecticide, a broad-spectrum insecticide, a fungicide and an oil, increasing phytotoxicity and compatibility risk while killing pollinators. Tank-mixing also makes it impossible to judge which product caused a problem. Keep the fungicide separate, skip the insecticides, and use neem oil only if a later aphid threshold is crossed. Remove severely affected leaves and avoid overhead irrigation in the evening.
3. What follow-up and record-keeping would you recommend, and what resistance-management principle applies?
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Re-scout 3–4 days after the fungicide spray, counting aphids on 20 randomly selected twigs and estimating disease severity on upper, middle and lower leaves. Record the date, crop stage, pest counts, beneficial insect numbers, product name, dose, water volume, area sprayed and pre-harvest interval. Note any phytotoxicity or bee mortality.
For resistance management, do not repeat the same fungicide mode of action in consecutive sprays. Mancozeb is a multi-site protectant with low resistance risk, but if a systemic is later needed for severe disease, rotate to a different group such as a triazole or a strobilurin-labelled product rather than using the same chemistry repeatedly. For aphids, preserve predators and use selective products only at threshold. Records should also show why an insecticide was withheld, because that decision is part of an IPM audit and helps the farmer defend the crop outcome.